Method and apparatus for controlling orientation of needle-like carbon particles in extruded carbonaceous stock

ABSTRACT

A means and method are provided for extruding compositions, particularly for extruding mixes of carbon and/or graphite particles and carbonizable binder. The means and method can additionally be employed to control the orientation of acicular particles in a composition during the extrusion thereof so as to control the properties of the extruded product. In all cases, the extrusion die&#39;&#39;s internal configuration is defined by at least three sections comprising first a converging section, then a diverging section and then a final section of substantially constant cross-section. A section of substantially constant cross-section may also sometimes follow the converging section. When employed in the extrusion of a carbonaceous mix containing needle-like carbon particles, dies of the present invention of appropriate design can be used to produce stock which after baking and graphitizing is characterized by having a transverse to longitudinal average coefficient of thermal expansion (CTE) ratio considerably less than that characteristic of corresponding stock in which the particles are all axially aligned.

United States Patent Bailey [451 May 30, 1972 [54] METHOD AND APPARATUSFOR CONTROLLING ORIENTATION OF NEEDLE-LIKE CARBON PARTICLES IN EXTRUDEDCARBONACEOUS STOCK [72] Inventor: Bruce L. Bailey, Lewiston, NY.

[73] Assignee: Great Lakes Carbon Corporation, New

York, NY.

[22] Filed: Nov. 7, 1969 [21] Appl.No.: 874,848

3,284,372 11/1966 Bailey 264/105 3,350,485 10/1967 Shesler et al...264/105 3,196,486 7/1965 Shesler et al... .264/176 1,700,208 l/l929Paisseau ..264/108 OTHER PUBLICATIONS Research and Development onAdvanced Graphite Materials" Volume XLll, Summary Technical Report, WADDTR 61- 72, Wright-Patterson Air Force Base, Ohio, August 1963 at 76 and167- 173.

Primary Examiner-Julius Frome Assistant Examiner.lohn H. MillerAttorney-Wallace F. Neyerlin [5 7] 1 ABSTRACT A means and method areprovided for extruding compositions, particularly for extruding mixes ofcarbon and/or graphite particles and carbonizable binder. The means andmethod can additionally be employed to control the orientation ofacicular particles in a composition during the extrusion thereof so asto control the properties of the extruded product.

In all cases, the extrusion dies internal configuration is defined by atleast three sections comprising first a converging section, then adiverging section and then a final section of substantially constantcross-section. A section of substantially constant cross-section mayalso sometimes follow the converging section.

When employed in the extrusion of a carbonaceous mix containingneedle-like carbon particles, dies of the present invention ofappropriate design can be used to produce stock which after baking andgraphitizing is characterized by having a transverse to longitudinalaverage coefficient of thermal expansion (CTE) ratio considerably lessthan that characteristic of corresponding stock in which the particlesare all axially aligned.

12 Claims, 2 Drawing Figures Patented May 30, 1912 3,666,847

2 Shouts-Sheet 1 BRUCE LBA-ILEY Patented May 30, 1972 2 Shoots-Sheet 2METHOD AND APPARATUS FOR CONTROLLING ORIENTATION OF NEEDLE-LIKE CARBONPARTICLES IN EXTRUDED CARBONACEOUS STOCK BACKGROUND OF THE INVENTION lField of the Invention This invention relates broadly to a method andapparatus for controlling the orientation of acicular particles in anextrudable material or composition so as to control the properties ofthe extruded product. The invention more particularly relates to theproduction and manufacture of carbon and graphite products and to aspecial means or apparatus particularly useful in carrying out one ofthe steps of said production and manufacture, viz. the extrusion step.

2. Description of the Prior Art In the conventional or normal type ofextrusion of green carbon bodies such as electrodes from a mixture ofcarbon and/or graphite particles and a binder, such as pitch, the mix issubjected to essentially continuous reduction in a converging die or diesystem connected directly to the mud cylinder of the press. In theproduction of certain graphite electrodes for steel furnaces, and othergraphite products, such as anodes for brine electrolysis and graphitefor nuclear reactors, a high percentage of the carbon and/or graphiteparticles employed in the mixture are frequently acicular or needle-likein shape. When extruding in a conventional die system, the reductionratio, that is, the ratio of the cross-sectional area of the mud chamberto the cross-sectional area of the product to be extruded is frequentlyso great that virtually all of the needle-like particles end up alignedwith their axes parallel to that of the product. This leads to agraphite product which has a relatively high coefficient of thermalexpansion (CTE) in the direction(s) perpendicular to, as compared toparallel with, the direction of extrusion and consequently to a graphiteproduct which may not give optimum performance in the particularenvironment in which the graphite product is to be used, for examplegraphite electrodes in service on a modern ultra-high-powered electricsteel furnace where the thermal shock and thermal stress conditions areparticularly severe.

To control the alignment or orientation of needle-like particles to thedegree that is required or desired in the final graphite electrode whenusing a conventional batch type extrusion press coupled with a singleconverging die, the degree of reduction during extrusion, or, in otherwords, the ratio of the cross-sectional area of the mud cylinder of thepress to the cross-sectional area of the product would have to be sosmall that, particularly in the case of large-diameter electrodes, onlyone plus a small-fraction electrode could be extruded from a givencharge to the press. This would mean that the extrusion operation wouldhave to be interrupted much more frequently than when employing aconverging die with a high degree of reduction and that at least everyother electrode extruded would contain a so-called batch interfacegenerated by the mating of two successive batches or charges to thepress. Such interfaces have frequently caused problems .in processing,especially when the degree of reduction through extrusion is small,since they tend to persist throughout subsequent processing andrepresent potential regions of weakness or stress concentration.

A conventional extrusion press with a large mud cylinder, because of itslarge cross-sectional area as compared to the cross-sectional area ofthe electrode to be extruded, permits the extrusion of more than oneelectrode from a given charge and thus minimizes problems connected withbatch interfaces between charges. This, however, suffers thedisadvantage of restricting the degree of freedom over the control ofthe grain orientation in the extruded product since a high degree ofreduction leads to substantially complete or axial alignment of theparticles.

Also, many conventional presses are of the tilting type and have anassociated vertical tamping apparatus. A tamping pressure higher thanthe extrusion pressure is beneficial in the attainment of a high densityin the final product. In such cases,

a separate mechanism is required for high pressure tamping in order toconfine the mix, otherwise, the tamping is limited to the normalextrusion pressure of the single die system, since, if this pressure isexceeded when the press cylinder is in the vertical position requiredfor charging and tamping, the material would begin to extrude.

Previous attempts have been made to devise extrusion methods andapparatuses which will accomplish some of the goals and objectives ofthe present invention such as controlling the transverse to parallel orlongitudinal (T/L) ratio of the CIE of the bodies produced, and U. S.Pat. No. 3,350,485 is illustrative of one type of such approach ordevelopment. Appendix VI (pages 167-172) of Technical Documentary ReportNo. WADD TR6l-72, Vol. XLII describes another related development.However, the techniques and solutions devised in these references aresubstantially different from the methods and apparatuses devised andemployed in the present invention.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a means and method for controlling the orientation of acicularparticles in a composition during the extrusion thereof so as to controlthe properties of the extruded product.

A further and more specific object of this invention is to provide ameans and method for controlling the orientation of the coke and/orgraphite particles and binder in a carbonaceous mix during extrusion soas to effect the desired prov perties in the final product, particularlythe thermal expansion properties. This latter objective of the presentinvention is especially applicable to the processing of coke and/orgraphite particles of high quality, i.e., to particles a high percentageof which contain or possess a needle-like structure.

Another object is to accomplish the foregoing while still employing aconventional capacity mud chamber and also, if desired, while stillemploying a tilting type press and associated tamping apparatus.

It is a finding of this invention that when processing such acarbonaceous mix containing needle-like coke and/or graphite particlesand a carbonizable binder in a generally longitudinal direction througha forming die of the present invention, the alignment of the particlescan be substantially altered such that the ratio of the transverse tothe longitudinal average coefficient of thermal expansion of theextruded stock, after baking and graphitizing, is reduced from thatcharacteristic of axial alignment. Graphite bodies with such reduced(T/L) CTE ratios or controlled thermal expansion properties may beadvantageous for nuclear reactor applications, for reasons discussed inthe aforesaid U. S. Pat. No. 3,350,485; they may also be advantageouswhen used as thermic electrodes in electric steel furnaces and in otherapplications.

It is an additional finding of the present invention that carbonaceousstock of generally improved structure and typically of higher strength,stemming primarily from the higher pressures that can be used inextruding the mix, can be produced when using process techniques andapparatus within the broad scope of the present invention, even whenlittle or none of the coke and/or graphite particles employed in the mixbeing processed is of the needle-like type.

The dies employed to accomplish any of the aforedescribed goals of thepresent invention are all characterized by possessing at least threesections (which sections are also preferably coaxially aligned)comprising first a converging section, then a diverging section and thena final section of substantially constant crosssection. A section ofsubstantially constant cross-section may also sometimes follow theconverging section. In the production of cylindrical products, thesections are each also preferably characterized by possessing smoothinterior contours and are also so shaped and designed that across-section at any location in each of the sections is circular. (Aswill be discussed hereinafter, however, the stock being extruded neednot always possess such a circular crosssection at any location.) Aspreviously indicated, there may also be a region or section ofsubstantially constant cross-section (e. g., a substantially cylindricalsection) located after the converging section although this requirementwill depend upon the particular die contour within each section and alsothe particular specific goal being sought at the time.

The dimensions and contours of the sections of the die are so regulatedas to develop the desired improved structure and physical properties.When the mix employed involves particles of the needle-like type, theseparticles are substantially aligned with their axes parallel to the axisof the extrusion cylinder upon exiting from the converging section,which is attached directly to the straight cylindrical barrel or mud potof the extrusion press. From this section, the material enters thediverging portion or expansion section of the die system where theindividual grains or needle-like particles are forced to take a positionsuch that their axes tend to become oriented in a plane that is at anangle to or perpendicular to the axis or direction of extrusion,depending upon the amount of divergence. The material then moves intothe section of substantially constant cross-section where theorientation of the grains or needle-like particles effected in thediverging section becomes fixed, so that on the average the angle ofinclination that the axes of these needle-like particles make with theaxis of extrusion is such that the desired CTE characteristics in thefinal product are achieved.

As will become clearer from a consideration of the Exampleswhich follow,the amount of change in the CT E characteristics in the final product isdependent upon the ratio of the cross-sectional area of the finalsection to the minimum crosssectional area of the converging section.This ratio is preferably at least 1.2 to 1 in order to effectsignificant realignment of the particles. The transverse CTE is largestwhen the ratio of the cross-section al area of the final section to theminimum area of the converging section is the smallest. An isotropic (Tand L CTEs substantially the same) carbon body is typically achievedwhen the cross-sectional area of the final section is about 2% times theminimum area of the converging section (2.5 to 1). As the ratio of thecross-sectional area of the final section to the minimum area of theconverging section is increased beyond the foregoing 2.5 to 1 ratio, theT CTE becomes smaller so that the T/L ratio also becomes smaller. Theproduction of carbon and graphite bodies possessing such reduced T/L CTEratios are desirable for certain specific applications just as arebodies which are substantially isotropic.

The contours of the internal surfaces of the converging or divergingsections of the die system are variable but generally consist of smoothsurfaces, continuously decreasing or increasing gradually, dependingupon whether the converging or diverging section is involved. Mostfrequently the cross-sectional shape of the material being extruded atany given location will be circular. However, in some cases it may bedesirable that the converging and/or diverging sections possess ordefine other geometrical configurations, such as to define rectangular,hexagonal or annular cross-sectional shapes.

The over-all length of the die system is limited mainly by practicalconsiderations. Since the structural integrity of the material exitingfrom the die system is preserved mainly by the frictional forces actingon its surface, the length (L of the section of substantially constantcross-section should be such as to exert the desired frictional force.The preferred length (L should not be less than twice the diameter (Dand more preferably, 3 or 4 times the diameter so as to insure that thefrictional forces provided by section C are sufficient to provide enoughrestraint to let the mix being extruded fully fill the final section.The length (L and design of the diverging section are matched to and/orcorrelated with the diameter and length of the final section. The designof the diverging section and the rate at which the material beingextruded is forced or passes therethrough should also be such that thematerial exiting from the converging section does not rifle" or shotgunthrough this expansion section thereby negating its function.

No baffle plates nor any other types of obstruction are placed withinthe die system, except perhaps a mandrel in case a material of annularcross-section is to be extruded, and the control of the alignment of theparticles (and consequently the achievement of the desired properties)are attained substantially entirely by means of the forces applied tothe mix by the internal configuration of the die system employed and ofthe various sections thereof.

BRIEF DESCRIPTION OF THE DRAWINGS A representative die of preferredconstruction is show in cross-section in FIG. 1. This figure also showsthe internal contours of the various essential sections of the die, aswell as auxiliary apparatus used with the die.

FIG. 2 also shows section contours, similar to those shown in FIG. 1, ofdies embraced within the invention. FIG. 2, however, is a schematicrather than a cross-sectional view.

DETAILED DESCRIPTION OF THE DRAWINGS & OF THE PREFERRED EMBODIMENTS InFIG. 1 the converging section is shown at A, the diverging or expansionsection at B, and the section of substantially constant cross-section atC. It will be noted that in section A, the die contour is continuouslydiminishing and that the cross-section at any point along its lengthdefines a circle (although the cross-sectional area is changing). Thesame feature applies also to section B, except that in section B, thecontour is frusto-conical and there is gradual expansion instead ofreduction.

It should be noted that the sections described as A, B and C refer moreto the contours of the die sections rather than the physical parts ofthe die system, these latter now being described in connection with thereference numerals of the drawing.

The mix to be extruded is charged into a cylindrical chamber of mudcylinder 1 of a conventional hydraulic or mechanical press. The mix isforced through section A by advancing ramhead 2, which is connected tothe mechanically or hydraulically actuated ram 20. (Such a means forforcing the mix into the die sections of the extrusion dies of thepresent in vention is greatly preferred over other means, such as ascrewtype extruder.) It will be appreciated that, as illustrated, only aportion of the mud cylinder is shown and that ram 20 and ramhead 2 haveneared the end of their stroke in the direction toward convergingsection A. The press tenninates in a flange la which is coupled withflange 3a of member 3 (which defines converging section A and alsodiverging section B) by means of ring clamp 4. Flange 3b of member 3 iscoupled to flange 5a at the inlet end of member 5, (which definessection C) by means of ring clamp 6. Section C is of substantiallyconstant cross-sectional area, e.g., cylindrical.

It should, of course, be appreciated that the manner in which thesections are joined together in FIG. 1 may be varied, as may also thenumber of members and clamps employed in order to obtain the desiredconfiguration of the entire assembly, the particular way the members arejoined and shown in FIG. 1 being illustrative only.

The internal configuration of the die shown in cross-section in FIG. 1is repeated schematically in FIG. 2 so as to further and more completelyillustrate dimensions and angles and curves being discussed, which areconsidered representative of the present invention.

The die of FIGS. 1 and 2 is employed in the production of a cylindricalgraphite product with a diameter of 24 inches. The mud cylinder" has adiameter of 40 inches, (diameter D,). In passing through section A,which possesses a length I of about 30 inches, the stock is graduallyreduced to a diameter of about 15 inches (D The cross-sectional areareduction ratio of this particular die, therefore is about 7 to l. Areduction ratio no higher than about 15.0 to l is normally employed inthe converging section. Diverging section B has an average angle ofdivergence of about 25 (angle x) with respect to the axis of the die,and the stock being extruded therethrough' changes from a diameter ofabout 15 inches at its inlet to a maximum of about 24 inches (D (across-sectional area expansion ratio of about 1 to 2.6), over its lengthL of about which follows, a die of such a particular configuration,i.e., one with a very high ratio between the area of the final sectionto the minimum area of the converging section, is used to effeet agreater reorientation of the needle-like particles. This is inches.(Angle x will generally be between about and 5 indicated by the lower TCT E and higher L CTE shown in Exabout 45.) The maximum diameter of 24inches is maintained ample 2A as compared to the corresponding values inExamfor a distance 1.. of about 72 inches in section C which is of P 2and substantially constant cross-section, e.g., cylindrically shapedOther die systems and configurations may also be employed member 5 (FIG.1 It will also be noted that in this particular in carrying out thepresent invention and in achieving various die design or system thecross-sectional area reduction ratio in objectives thereof. Theforegoing described die systems, the converging section is about 2.7times the expansion ratio therefore, are not intended to be [imitativebut rather to be ilof the cross-sectional areas of the stock enteringand leaving lustrative only of the die configurations and dimensionswhich the diverging section. may be used in the present invention.

It should also be noted that the cross-sectional area of the 15 In mo tin tances, the minimum cross-sectional area of the stock leaving thisparticular die is about 2.55 times the cross- 1 to k emerging from the etion of substantially constant sectional area of the stock when itreaches its minimum cross-section will be at least 6 square inches.Extruded stock, dimensions in the converging section. As will be clearfrom e.g., carbonaceous, between about 90 and about 3,000 square Example2 of the Table which follows, such a relationship is inches incross-sectional area is typical of the products necessary in the dies ofthe present invention which are produced in the present invention.specifically designed for the processing of a carbonaceous mixProperties of graphitized products which have been excontainingneedle-like particles and wherein it is desired to: truded throughconverging-diverging dies of the present inalter the alignment of saidparticles to such an extent that the vention compared to graphitizedproducts or controls" ratio of the transverse to the longitudinalaverage coefficient whi h have been conventionally extruded (i.e., usinga Stanof thermal expansion of the stock, after baking and graphitizdar"single converging section) are set forth in the following mg, isreduced, from that characteristic of axial alignment, to; Table. In theExamples of the Table, the carbon aggregate approximate unity. consistedof 45 parts of particles, ranging in Tyler screen size Another diesystem embraced within the present invention,j from through 3 mesh tojust under 20 mesh, and 55 parts of and one which is particularlyadvantageous for the production flour milled to a fineness of 55 percentthrough 200 mesh. In of large diameter stock without having to resort tolarger mud ea h of the Examples, the carbon aggregate was mixed withcylinder and extrusion presses, is one wherein the maximum the indicatednumber of parts of binder (coal tar pitch) having diameter of the stockleaving the diverging section exceeds its 1 'a softening point of about1 10 C, and was extruded at the indiameter in the mud cylinder 1. Forexample, the die system dicated pressure, (Other carbonizable binderswhich may be may possess an inlet 40 inches in diameter and the diameterof 1 used in the present invention include resins, tars, petroleum thestock may be caused to change from 40 inches (D to 15 35 pitches andresidues. Mixtures of binders may also be used.) inches (D in convergingsection A (a cross-sectional area The extruded products were then bakedand graphitized (to reduction ratio of about 7.3 to 1), and finally toinches (D about 2,600 C) under identical conditions conventional in inpassing through diverging section B (a cross-sectional area the art.

TABLE Example 1 2 2A 3 4 Die Standard di i g i g i rs i g Standard i x lg i n g Dimensions (inches) of DiD D; 5, Q525 1 5252; 2 :5; I 52 52Cross-sectional area ratios a D g Dz/D D12)22 D12/ Type coke Needk Needk: N cedl: Needle Need? Binder level (pts./100 pts. coke)- 25. 6 25.6 25. 6 27.4 27. 4 Extrusion pressure (p.s.i.) 1, 600 1, 700 1, 700 1,600 1, 800 Properties of graphite:

Apparent density (g./m1.) 1. 61 1. 61 1.61 1. 58 1. 58 Electricalresistivity (lengthwise) (ohm-in.X10 27 35 42 31 37 CTE C.-1 (20100 C.)2

expansion ratio of about 1.0 to 11.1, whereafter its diameter remainsconstant in passing through section C.

In this die system, the average angle of convergence in section A is thesame as that of section A of FIGS. 1 and 2, unless length L is greatlyreduced. The diverging angle (x) employed in section B may besubstantially identical to that of FIGS. 1 and 2,, but this will dependupon and be related to the dimension L of the die in effecting thediameter change indicated, i.e., from 15 inches to 50 inches.

In this embodiment of the invention it will also be noted that the finaldiameter D of the stock (50 inches) exceeds the diameter D (40 inches)of the stock entering the converging section. As shown by the results ofExample 2A in the Table It will be noted that the dies of the presentinvention (Examples 2, 2A and 4) lessened the alignment of the particlesso coefficient of thermal expansion (T/L ratio) of the stock, afterbaking and graphitizing, was reduced from that characteristic of axialalignment (viz. from that which resulted when a Standar extrusion diewas used, as in Examples 1 and 3).

The die system of this invention is operated in much the same manner asthat employed by those skilled in the art in extruding throughconventional dies. As previously pointed out, however, caution must beobserved to adjust conditions so that the material in the system flowssmoothly and does not shot-gun or rifle through the expansion section.Extra care must also be observed in closely controlling those operatingvariables that affect the rheological characteristics of the mix inorder to assure an axially symmetrical pattern of flow through thesystem as successive batches are charged to the system. A multiplicityof separately controlled heating coils may be employed to surround thevarious sections of the die to assist in this control.

The mix is charged to the mud chamber of the press and is consolidatedby advancing the ram until the die system is filled. (To start with itis necessary to block off the exit end in order to completely fill theexpansion portion of the die system.) Once the die is filled theextrusion can be conducted in the usual manner.

As previously indicated, when using a mix composed of needle-likeparticles and a coal tar pitch binder, and dies with design features aspreviously described, the particles tend to be oriented in the mudchamber of the press with their long axes perpendicular to the axis ofthe cylinder or direction of motion. As the mix progresses through theconverging section, the needle-like particles become aligned with theirlong axes mutually parallel and parallel with the axis of the die. Thenas the mix progresses into the diverging section, the needle-likeparticles tend to become realigned with their axes generally inclined atsome angle with respect to the axis of the die system depending upon theamount of divergence. Finally the mix progresses through the finalsection of substantially constant cross-section, where little or nofurther reorientation of the particles occurs.

Essentially then, one of the main purposes of the dies embraced withinthe present invention is to permit the control of grain orientationduring extrusion as a means of controlling the coefficient of thermalexpansion (CTE) characteristics in the final graphite product. It shouldbe emphasized that the level or magnitude of the electrode CTE will bedependent upon the CTE characteristics of the raw material, and that theratio of the transverse to the longitudinal CTE will be dependent notonly on the raw material but also on the specific dimension of thevarious sections of the die system.

It should also be appreciated, of course, that the dies of the presentinvention can also be used for achieving the use of increased extrusionpressures and/or the production of larger cross-sectional area stockthan that of any particularly sized mud cylinder that might be on handand that for such purpose(s) the dies can also be used withnon-needle-like particle mixes.

I claim:

1. A method for controlling the thermal expansion properties ofcarbonaceous stock having a minimum cross-sectional area of at least 6square inches comprising the step of forcing, by means of a conventionalhydraulically or mechanically actuated ram press, a carbonaceous mixcontaining needle-like coke particles or needle-like graphite particlesor mixtures thereof and a carbonizable binder through a forming diewhich is free from any obstructions and which possesses at least threesections defined by the walls of the die comprising first a convergingsection, then a gradually diverging section possessing no abrupt contourchanges, and then a final section of substantially constant crosssection, but whose length is not less than twice its diameter, thecross-sectional area of the final section also being at least 1.2 timesthe minimum crosssectional area of the converging section, whereby thealignment of said particles is altered to such an extent that the ratioof the transverse to the longitudinal average coefficient of thermalexpansion of said stock after baking and graphitizing is reduced fromthat characteristic of axial alignment.

2. A method according to claim 1 wherein the sections of said formingdie are coaxially aligned.

3. A method according to claim 1 wherein the average angle of divergencein the diverging section is between about 15 and about 45 with respectto the axis of the die.

4. A method according to claim 1 wherein said converging section isfollowed by a section of substantially constant crosssection before thediverging section.

5. A method according to claim 1 wherein the cross-section of the mixbeing extruded at any location in each of the sections is circular.

6. A method according to claim 1 wherein the cross-sectional area of thefinal section of the forming die is about 2% times the minimumcross-sectional area of the converging section.

7. An apparatus for extruding comprising a cylindrical chamber intowhich the material to be extruded is charged, a conventionalhydraulically or mechanically actuated ram press for forcing theextrudable material through the apparatus, and a forming die leadingfrom the outlet of the cylindrical chamber, said forming die beingcharacterized by being free from any obstructions and by possessing atleast three sections defined by the walls of the die comprising first aconverging section, then a gradually diverging section possessing noabrupt contour changes, and then a final section of substantiallyconstant cross-section, but whose length is not less than twice itsdiameter.

8. An apparatus according to claim 7 wherein the sections of saidforming die are coaxially aligned.

9. An apparatus according to claim 7 wherein said converging section ofthe forming die is followed by a section of substantially constantcross-section before the diverging section.

10. An apparatus according to claim 8 wherein said coaxially alignedsections of the forming die are each also characterized by possessingsmooth interior contours and wherein a cross-section at any location ineach of the sections is circular.

11. An apparatus according to claim 7 wherein the crosssectional area ofthe final section of the forming die is at least 1.2 times the minimumcross-sectional area of the converging section.

12. An apparatus according to claim 11 wherein the crosssectional areaof the final section of the forming die is about 2% times the minimumcross-sectional area of the converging section.

2. A method according to claim 1 wherein the sections of said formingdie are coaxially aligned.
 3. A method according to claim 1 wherein theaverage angle of divergence in the diverging section is between about15* and about 45* with respect to the axis of the die.
 4. A methodaccording to claim 1 wherein said converging section is followed by asection of substantially constant cross-section before the divergingsection.
 5. A method according to claim 1 wherein the cross-section ofthe mix being extruded at any location in each of the sections iscircular.
 6. A method according to claim 1 wherein the cross-sectionalarea of the final section of the forming die is about 2 1/2 times theminimum cross-sectional area of the converging section.
 7. An apparatusfor extruding comprising a cylindrical chamber into which the materialto be extruded is charged, a conventional hydraulically or mechanicallyactuated ram press for forcing the extrudable material through theapparatus, and a forming die leading from the outlet of the cylindricalchamber, said forming die being characterized by being free from anyobstructions and by possessing at least three sections defined by thewalls of the die comprising first a converging section, then a graduallydiverging section possessing no abrupt contour changes, and then a finalsection of substantially constant cross-section, but whose length is notless than twice its diameter.
 8. An apparatus according to claim 7wherein the sections of said forming die are coaxially aligned.
 9. Anapparatus according to claim 7 wherein said converging section of theforming die is followed by a section of substantially constantcross-section before the diverging section.
 10. An apparatus accordingto claim 8 wherein said coaxially aligned sections of the forming dieare each also characterized by possessing smooth interior contours andwherein a cross-section at any location in each of the sections iscircular.
 11. An apparatus according to claim 7 wherein thecross-sectional area of the final section of the forming die is at least1.2 times the minimum cross-sectional area of the converging section.12. An apparatus according to claim 11 wherein the cross-sectional areaof the final section of the forming die is about 2 1/2 times the minimumcross-sectional area of the converging section.